Insulated alkali metal conductor

ABSTRACT

ELECTRICAL CONDUCTORS COMPRISING AN ALKALI METAL CONDUCTING CORE ARE COATED WITH A PREFORMED HEAT SHRINKABE INERT OLEFIN HYDROCARBON POLYMER FILM OR SHEET AND THE HYDROCARBON SHRUNK INTO TIGHT ENGAGEMENT WITH THE ALKALI METAL. THE HEAT OF THE MOLTEN METAL OR OF THE METAL WHILE IT IS COOLING CAN BE USED TO SHRINK THE POLYMER.

May 18, 1971 A. GUTTAG INSULATED ALKALI METAL CONDUCTOR 2 Sheets-Sheet 1 Original Filed Sept. 25, 1967 Ill lll/[Ill] l//I/ IIIIIIIIlIl/lllI/Ill W mvmon Hz v//v far/wf BY mwQ/m- ATTORNEYS May 18, 1971 A. GUTTAG INSULATED ALKLI METAL CONDUCTOR 2 Sheets-Sheet 2 Original Filed Sep. 25, 1967 LAL United States Patent O'ce 3,579,624 Patented May 18, 1971 3,579,624 INSULATED ALKALI METAL CONDUCTOR Alvin Guttag, Bethesda, Md., assignor to Weston Chemical Corporation, New York, N.Y. Original application Sept. 25, 1967, Ser. No. 670,166. Divided and this application Mar. 7, 1969, Ser. No. 828,037

Int. Cl. B29c 27/20; B29f 3/10; D01d 5/16 U.S. Cl. 264-171 19 Claims ABSTRACT OF THE DISCLOSURE Electrical conductors comprising an alkali metal conducting core are coated with a preformed heat shrinkable inert olefin hydrocarbon polymer film or sheet and the hydrocarbon shrunk into tight engagement with the alkali metal. The heat of the molten metal or of the metal while it is cooling can be used to shrink the polymer.

An alkali metal conductor having a coating of an inert olefin hydrocarbon polymer has the surface of the coating modified to render it receptive to a vinylidene chloride polymer and then a vinylidene chloride polymer is directly integrated into said modied surface to render the olefin hydrocarbon polymer substantially impervious to oxygen and carbon dioxide.

This is a division of application Serial No. 670,166, led Sept. 25, 1967, now U.S. Pat. No. 3,463,872, dated Aug. 26, 1969.

The present invention relates to electrical conductors comprising a conducting element of an alkali metal continuously surrounded by a layer of a normally solid hydrocarbon polymer.

Recently it has been proposed in Humphrey patents 3,333,037 and 3,333,049 to prepare alkali metal conductors having a covering of an olefin polymer. The polymer can be preformed in tubular form and the molten sodium carefully fed into the tube. More preferably the molten hydrocarbon polymer is extruded around the alkali metal, preferably sodium, while the molten metal is also being extruded. This procedure, however, presents several problems. In the first place the molten polymer as it is extruded is very weak and breaks can occur. Secondly, the molten polymer has a softening point above 100 C., generally at least 105 C. and often higher, whereas sodium has a soliditication point of 97.8 C. As is well known sodium contracts considerably as it is cooled. Thus the density of sodium increases as it goes from a liquid at about 97.8 C. with a density of 0.929 to a solid at 97.8 C. with a density of 0.952 and to a solid at C., with a density of 0.971. As a result there is a tendency for a, space to form between the solidified sodium and the surrounding polyethylene or the like coating, e.g. in tubular form. Furthermore regular polyethylene has a tendency to tear easily and has a relatively low tensile strength, e.g. around 2000 to 2600 p.s.i. at room temperature while the tensile strength is reduced sharply at elevated temperatures. Humprey patent 3,333,049 discloses that after the polyethylene tube is filled with sodium the product can be passed through a series of dies on a conventional Wire drawing machine to increase the ultimate tensile strength of the conductor. This process of course has the danger that the polyethylene iilm might break during the drawing operation particularly since the thickness of the polyethylene covering is reduced.

It has also been proposed in Humphrey patent 3,333,- 050 to extrude a monooleiin polymer, c g. polyethylene, around sodium to form a coating or liner around the sodium and to extrude a polymer reactive with sodium around the polyethylene. The reactive polymer can be a material such as polyvinyl chloride or can be an olefin polymer, e.g. polyethylene containing a crosslinking agent such as a peroxide which is reactive with sodium. The Humphrey patent shows that to cure the peroxide treated polyethylene it is necessry to employ an oven at a temperature of 190 C. for at least 10 minutes. This of course is well above the softening point of polyethylene and consequently problems are encountered in retaining the initial liner uniformly around the sodium. The problem is increased by the fact that the temperature of curing is also well above the melting point of the sodium and not only will the sodium expand about 50% but form unstable sodium will be within form unstable polyethylene. Furthermore it is necessary by such as procedure to form a laminate with the attendant dangers of delamination.

When using an all hydrocarbon insulation there is the further problem that it is relatively porous to oxygen, a gas known to be reactive with sodium and other alkali metals. Thus low density polyethylene (0.914-0.92) has an oxygen permeability of 500 expressed as cc./ 100 sq. in /mil/24 hours at 25 C., medium density polyethylene (093-094) has a permeability of 535, high density polyethylene (OSS-0.96) has a permeability of 185 while polypropylene (unoriented) has a permeability of and 370 oriented. On the same scale the carbon dioxide permeability for the low density polyethylene is 2700, medium density polyethylene 2500, high density polyethylene 580, polypropylene (unoriented) 150 and oriented polypropylene 180. v

Accordingly it is an object of the present invention to insure that a monoleiin polymer coating for an alkali metal adheres to the metal without any intervening air space.

A further object is to reduce the risk of breakage of an insulating hydrocarbon covering for an alkali metal electrical conductor.

An additional object is to prepare an alkali metal electrical conductor with an olefin polymer covering having reduced permeability to oxygen and carbon dioxide.

Yet another object is to provide an alkali metal electrical conductor with an olefin polymer covering as a protective layer for an insulating layer reactive with the alkali metal while at the same time avoiding any possibility of delamination between the olefin polymer and the reactive layer.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are gi'ven by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has now been found that these objects can be attained in several ways as more fully set forth belofw.

The entire disclosure of the Humphrey patents 3,333,- 037; 3,333,049 and 3,333,050 is hereby incorporated by reference.

While the following description Iwill make reference to sodium as the alkali metal, and this is the preferred form of the invention, the invention is also applicable to the other alkali metals having an atomic weight of less than 40, i.e. lithium and potassium.

According to one aspect of the invention the alkali metal electrical conductor, e.g. sodium, is provided with an insulating layer of a preformed heat shrinkable oriented monoolefn polymer. The orientation can be uniaxial or biaxial, most preferably biaxial. As the monoolefin polymer there can be used polyethylene, polypropylene, ethylene-propylene copolymer (eg. 50:50 by weight), ethylene-neohexane copolymer (e.g. :10 by weight), ethylene-deoene copolymer (:5 by weight),

-tube This aids in ensuring that the polymer remains in tight engagement with the metal while the latter cools and contracts. If crosslinked heat shrinkable polymer is used there is no danger of melting the polymer. When thermoplastic polymers are employed, e.g. irradiated polyethylene or polypropylene, the shrinking of the polymer should be carried out below the melting or softening point of the polymer. Thus molten sodium at 100 C. can be poured into irradiated biaxially oriented polypropylene or unirradiated uniaxially oriented high density (0.96) polyethylene tubng and the tubing will heat shrink about the metal into tight continuous engagement therewith.

The sodium in another example was cooled to room temperature and the solid sodium then introduced into a or irradiated biaxially oriented polyethylene (stretched at 90 C. 3:1 longitudinally and 3:1 laterally after irradiation to a megarad dosage). The tube had a diameter 10% greater than that of the sodium rod. After introducing the sodium the tube was heated to 90 C. with a blast of hot nitrogen from an air gun to shrink the tube in tight continuous engagement with the sodium.

One of the advantages of the present invention is that the heat shrinkable jacket if applied as a preformed tube can be larger in diameter than the sodium rod which is being extruded and there is no need to maintain a narrow pressure range on the alkali metal in order to maintain the shape of the tubing.

For safety purposes wherever the sodium or other alkali metal is exposed to the surrounding atmosphere the atmosphere is inert, e.g. an inert gas such as nitrogen, helium, argon or neon. When a coolant is employed it also should be inert to the alkali metal and the insulation. Thus it can be an inert gas as specified above or a hydrocarbon oil, eg. an aromatic or aliphatic hydrocarbon or cooling can be accomplished by passing a heat exchange liquid counter-current through a tube adjacent to the plastic tube containing the sodium or other alkali metal.

To reduce the oxygen and carbon dioxide permeability of the hydrocarbon polymers, and to a lesser extent to reduce the water vapor permeability, the hydrocarbon insulation can have an integral continuous coating of saran on the outside surface (i.e. the surface not in contact with the alkali metal conductor). As the insulation which is integral with the saran there can be employed any of the heat shrinkable monoolefin polymers set forth above or there can be used a non heat shrinkable monoolefin polymer such as the polyethylene, polypropylene or the like obtained by extruding the monoolefin polymer around the alkali metal conductor as shown in Humphrey patents 3.333.050; 3,333,049 and 3,333,037.

Unless otherwise indicated all parts and percentages are by weight.

By the term Saran there is intended as is well known in the art vinylidene chloride polymers containing at least 50% of vinylidene chloride and preferably at least 70% but not over of vinylidene chloride. There can be used copolymers, terpolymers, tetrapolymers and the like. Typical examples of copolymers are vinylidene chloride-acrylonitrile (75:25, 80:20, 85:15), vinylidene chloride-ethyl acrylate (80:20), vinylidene chloride-vinyl chloride (80:20), vinylidene chloride-vinyl chloride-dimethyl maleate (75 :20:5), vinylidene chloride-acrylonitrile-isobutylene (70:25 :5 vinylidene chloride butyl methacrylate (90: 10), etc.

The saran coating cannot be applied in the manner set forth in Humphrey patent 3,333,050 for other coatings since the thus extruded saran will not be integral with the polyethylene and there will be the problem of delamination pointed out previously.

Instead the saran can be applied as a solution in a solvent such as acetone, methyl ethyl ketone, tetrahydrofuran, methyl isobutyl ketone, ethyl acetate, butyl acetate, amyl acetate, nitromethane, nitroethane, 2-nitropropane, etc. The saran is normally present as a 10-20% solution but can be present in lesser amount or in some instances in greater amount.

The saran is preferably employed as Saran F-120 (a vinylidene chloride-acrylonitrile copolymer :20 having a viscosity of 200 cps. as a 20% acetone solution) as a 15% solution in a mixture of acetone and methyl ethyl ketone (3: 1).

The monoolefin polymers, e.g. polyethylene and polypropylene (whether irradiated or not and whether heat shrinkable or not) are normally not receptive to receive the saran. Accordingly the surface of the monoolefin polymer must be modified as is known in the art to render the surface receptive. Such modification can be done by treating the surface of the polyethylene or polypropylene (whether irradiated or not and whether heat shrinkable or not) with an oxidizing means, e.g. of the oxygen type such as chromic acid, aqua regia, sodium dichromate, potassium permanganate, sodium permanganate or an oxidizing gas flame or by treatment of the surface with corona discharge. Of course if heat is employed in the treatment of heat shrinkable polymer, e.g. an oxidizing gas flame is employed, the polymer should be maintained under tension until it has cooled below the shrink temperature. The time for this treatment is normally very brief since film travelling at a rate of to 600 ft./min. can pass under an oxidizing gas ame or under corona discharge for only a fraction of a second and have an adequate treatment.

After the surface of the monoolefin polymer has been modified in the manner indicated above (the preferred modification being with corona discharge) the surface is coated ywith the saran solution in a volatile solvent in order to provide the monoolefin polymer with a vinylidene chloride polymer (satan) directly integrated with the modified surface of the monoolefin polymer. The solvent is then removed, eg. in an oven. The Saran can be applied to the monoolefin polymer (e.g. polyethylene either irradiated or unirradiated) prior to application of the polymer as insulation to the alkali metal. Alternatively the saran can be applied to the monoolefin polymer after the polymer has been applied as insulation to the alkali metal. In the latter case the solvent employed preferably should be removable at a temperature below the melting point of the metal, e.g. sodium. Thus solvents such as acetone, methyl ethyl ketone and tetrahydrofuran are quite suitable for such purpose since they all boil below 95 C. The solvent appears to help the Saran penetrate slightly into the monoolefin polymer surface and thus obtain the directly integrated or merged relationship in contrast to a simple lamination.

The saran coating desirably can be from 0.075 mil to 1.5 mil but in no case should be more than 10% of the thickness of the monoolefin polymer. This is important for several reasons. Thus there is no danger of the Saran penetrating to the extent that it will Contact the sodium with which it is reactive. Furthermore, if the saran coating is too thick the low temperature flexibility of the insulation is greatly reduced and in addition the insulation tends to be too brittle.

Preferably the saran coating is about 0.1 to 0.3 mil thick. Despite the extreme thinness of the saran coating it has a great effect on the oxygen and carbon dioxide transmission of the coating. Thus a 0.2 mil integral Saran F- coating on a 5 mil film of polyethylene (10.914 density) insulation coating on sodium reduced the oxygen permeability of the polyethylene to 20% of its former value, and reduced the carbon dioxide permeability to 10% of its former value. A 1.5 mil integral coating of Saran F-120 (stretched 300% in each direction) heat shrunk on sodium on a 15 mil sheet of biaxially oriented polypropylene reduced the oxygen permeability of the polypropylene to less than 10% of its former value.

A 0.2 mil integral Saran F-120 coating on a 3 mil film of biaxially oriented irradiated polyethylene (0.914 densof 0.2 mil, the irradiated biaxially oriented polyethylene a thickness of 6 mils and the sodium metal a diameter of 112 mils.

Greater thicknesses of the saran layer can be built up by repeated passages through the saran bath.

In the form of the invention illustrated in FIG. 6 instead of employing heat shrinkable monoolelin polymer there can be used non heat shrinkable monooleiin lpolymer such as the regular 0.93 density polyethylene extruded as shown in FIG. of Humphrey patent 3,333,049. However, as previously indicated, preferably there is used the heat shrinkable monooleiin polymers. In the example i1- lustrated by the drawings the irradiated polyethylene had a density of 0.916 and the polypropylene had a density of 0.90. However, as previously pointed out other heat shrinkable polyethylenes and polypropylenes can be used.

The metallic sodium conductor as indicated can be circular, hexagonal, rectangular or of other cross-section.

As used in the present claims hot metal is intended to include metal having a temperature of 50 C. or above, e.g. 90 C., 100 C., 200 C. or even higher.

What is claimed is:

1. A method of preparing an electrical conductor having as essential components thereof an electrically conductive member of a solid alkali metal core and a flexible, partially oriented heat shrinkable hydrocarbon polymer layer of a monoolen having from 2 to 6 carbon atoms, electrically insulating and surrounding said conductive member, comprising feeding an alkali metal core into a continuous preformed and at least laterally oriented and heat shrinkable tubular iilm of the said polymer of a monooleiin, heating the said tubular iilms, allowing the said tubular lm to be partially relaxed and heat shrunk into tight continuous engagement with the alkali metal core allowing the shrunk tubular iilm and alkali metal core to cool.

2. A method according to claim 1 wherein the alkali metal core is in molten condition and the heat therefrom is employed to heat shrink the tubular iilm therearound the alkali metal core.

3. A method according to claim 1 wherein the alkali metal is in solid condition and at a temperature of at least 50 C. and the heat therefrom is employed to heat shrink the tubular film around the alkali metal core.

4. A method according to claim 1 wherein the heat shrunk tubular film surrounding said alkali metal core is coated with saran and the saran is applied after the tubular film has been shrunk onto the said alkali metal core.

S. A method according to` claim 1 wherein the tubular lim is a crosslinked polymer of a monooleiin having 2 to 3 carbon atoms.

6. A method according to claim 5 wherein the crosslinked polymer is polyethylene irradiated to an extent of 2 to 50 megarad.

7. A method according to claim 6 wherein the irradiated polyethylene is biaxially oriented.

8. A method according to claim 1 wherein the tubular lm is a heat shrinkable crosslinked polymer of a monoolefn having 2 to 3 carbon atoms integrally coated with saran and the saran has a thickness of `0.1 to 1.5 mils.

9. A method according to claim 8 wherein the monooleiin polymer is polyethylene irradiated to an extent of 2 to 50 megarad.

10. A method according to claim 9 wherein the irradiated polyethylene is biaxially oriented.

11. A method accoridng to claim 8 wherein the polymer is polyethylene.

12. A method according to claim 8 wherein the polymer is polypropylene.

13. A method according to claim 1 wherein the tubular film is biaxially oriented.

14. A method acocrding to claim 1 wherein the tubular ilm is uniaxially oriented in the lateral direction.

15. A method according to claim 13 wherein the shrinkage capacity of the oriented tubular film is from 3 to 75% at 97.8 C.

16. A method according to claim 1 wherein the alkali metal core and tubular tilm therearound are heated to heat shrink the tubular film.

17. A method according to claim 1 wherein the tubular iilm continues to heat shrink as the alkali metal core cools and remains in tight engagement with the cooling and contracting alkali metal core.

18. A method according to claim 1 wherein the alkali metal core is in a hot solid condition and at a temperature above 50 C. and the tubular iilm is of greater diameter than the said core and the hot core heats the tubular film and heat shrinks the tubular film into a continuous tight engagement.

19. A method according to claim 18 wherein the heat shrunk tubular iilm and core are cooled to room temperature and the tubular -iilm is briey heated to further heat shrink the tubular film.

References Cited UNITED STATES PATENTS 2,821,155 l/1958 Seckel 264-230 3,022,543 2/ 1962 Baird, Ir. et al. 264-95 3,093,448 6/ 1963 Kirkpatrick et al. 264-230 3,333,049 7/1967 Humphrey et al 264-171 3,333,050 7/1967 Humphrey et al 174-120 ROBERT F. WHITE, Primary Examiner I. R. THURLOW, Assistant Examiner U.S. Cl. X.R. 

